1. Field of the Invention
The present invention concerns a copying process including a "read" scan of an original and a "write" scan utilising a laser beam modulated in accordance with the "read" scan. The invention is applicable to the production of flexographic rollers by laser engraving, or gravure cylinders by laser engraving or by a photo-resist method and to general copying.
2. Description of the Prior Art
It is known to engrave flexographic printing rollers by means of a laser beam, wherein a master roller carrying the original artwork is mounted coaxially with, and on the same drive shaft as, a printing roller which latter is covered with a special polymeric material that can be cut sharply by a laser beam without deleterious edge damage effects. The original artwork is transferred to the printing roller surface either by optical scanning or by a system employing a copper mask.
Optical scanning involves the use of a master roller and a printing roller of identical dimensions rotated together. A photo-electric scanning device provides an output from its scan of the artwork to pulse the laser. This method suffers from the disadvantages that either the rollers must be moved smoothly and continuously past the scanning device and the laser, or (where the rollers are particularly heavy) the device and the laser must be moved past the rollers. This mechanical arrangement results in an elongated, cumbersome apparatus requiring a great deal of space. Further, problems of inertia arise when starting or stopping the rotary and traversing movements of the relatively heavy objects involved. Also it is difficult accurately to align adjacent pieces. Moreover, where geared drives are used, fluctuations in speed at tooth frequency inveitably arise. Finally when it is desired to replace the rollers by rollers of different dimensions, the replacement procedure is cumbersome.
The mask system requires the preparation of a mask by a photo-etching technique resulting in a pattern of copper foil and plastics film unprotected by copper, to correspond to the original artwork. The mask is then placed on the printing roller. The copper reflects the laser beam while the plastics film is consumed and the beam can then attack the polymeric roller surface. This method has nearly all of the shortcomings of the optical scanning system mentioned above, and has the additional defect of involving the extra step and expense of requiring a mask to be in the first place; furthermore, the mask cannot be re-used.
The above mentioned disadvantages of both methods are serious enough; however, perhaps their most important and common defect is that they require a great deal of expensive, time-consuming labour by skilled operators before a jointed printing surface that appears seamless (endless) can be produced from an unjointed artwork original.